I think my case might help you:
Im using a Nforce3 board with three dimm sots (made for dual C). Im running 2 x DDR400 and can go up to 228 FSB this way - stable (tested with Prime95 blend 24h). I too, just like you, have an extra DDR400 stick that I decided to put in the remaining slot. I had to decrease the OC back to 200FSB in order to finnish the blend test with no errors. Still, POST screen, CPUZ, AIDA, Everest all report Im running in DC (which is impossible AFAIK). I also tried testing both settings with photoshop which I am using daily and dint find any big differences (guess I aint working with realy huge files). So my advice would be:
- If you are running processor intensive applications, stick with two sticks in DC - try getting the FSB as high as you can
- If you are running memory intensive applications (PS, graphics in general), I would give all three sticks a chance. Mind though, that (for example) PS uses a lot of CPU when processing data so I guess its a tough call...
Do as folks told you - bench, test the system with the programs you're using daily - a cool tip is to use a stop watch to time how fast certain applications start, photos open, archives unzip/rar etc...(you get the idea).
Btw, DC or SC? AFAIK, on nforce boards DC only brings up up to 2,3 % so...:/